The present invention relates generally to the field of rechargeable batteries, and more specifically to a cell design, electrolyte formulations and reconditioning procedures for making electrochemical cells and multi-cell batteries. Still more particularly, this invention relates to an electrolyte flow-assisted nickel zinc cell construction which is capable of much greater cycle life than cells of the prior art.
Efforts to develop the nickel-zinc (Ni—Zn) battery system date back more than 100 years, with many unsuccessful attempts made to commercialize it. The net stoichiometry of the battery on discharge is given by the equations set forth below:
E02NiOOH + 2H20 + 2e− → 2Ni(OH)2 + 2OH−0.49 VZn + 2OH− → Zn(OH)2 + 2e−1.24 V2NiOOH + 2H20 + Zn → 2Ni(OH)2 + Zn(OH)21.73 V
Features of the Ni—Zn system that have made such batteries attractive in the field include: 1) A high theoretical specific energy of 334 Wh/kg; 2) Good power capability (due to the rapid kinetics of the electrodes and low resistance of the electrolyte); 3) Relatively low cost anode material (zinc); and 4) Safe, non-flammable electrolyte. With respect to the low cost of the anode material, based on current metals prices (nickel at $5.58/lb, and zinc at $0.65/lb), the theoretical nickel and zinc metals cost of the system is only about $16.61/kWh.
However, there are several historical drawbacks of the system, which have precluded its widespread adoption. These generally involve shortcomings of the cycle life of the zinc electrode caused by material migration/shape change and dendritic shorting. In particular, the zinc electrode in nickel-zinc battery systems has a tendency to become misshapen due to anisotropic growth of the zinc deposited on the electrode during repeated charging.
To reduce shape change, many approaches have been tried with varying degrees of success, including modifications to the electrolyte, the zinc electrode design, or the cell design. These approaches generally involve reducing either the solubility or the concentration gradients of the zinc in the electrolyte. For example, U.S. Pat. No. 4,358,517 to Jones and U.S. Pat. No. 5,863,676 to Charkey et al. disclose methods involving the use of calcium oxide or hydroxide additives to the zinc electrode.
Another approach to improving cycle life involves modifications to the battery electrolyte. In this regard, many different additives to the electrolyte have been tried. The modifications to the electrolyte typically have as their object to reduce the solubility of zinc, and thereby reduce shape change. Typical examples of this approach include fluoride/carbonate mixtures, as disclosed in U.S. Pat. No. 5,453,336 to Adler et al., and borates, phosphates, and arsenates mixtures, as disclosed in U.S. Pat. No. 5,215,836 to Eisenberg.
To reduce the likelihood of dendritic shorting, micro-porous barrier films, positioned between the electrodes, have been tried. Most recently, micro-porous polyolefin separators, (e.g. Celgard® battery separators), have had some success, but these materials are quite expensive.
A sealed starved mode of cell operation is also thought to be beneficial with respect to elimination of dendrites. In this case, oxygen generated on the overcharge of the positive nickel oxide electrode is thought to oxidize metallic zinc dendrites. Since all zinc electrodes will evolve small amounts of hydrogen gas on standing, some means of oxidizing hydrogen is also needed in a sealed cell, or else the cell pressure will increase without limit.
L. Zhang, J. Cheng, Y. Yang, Y. Wen, X. Wang, and G. Cao, “Study of zinc electrodes for single flow zinc/nickel battery application,” Journal of Power Sources, vol. 179, pp. 381-387 (2008) describes a nickel zinc battery with flowing electrolyte to reduce the formation of dendrites on the zinc electrodes. However, the flow rates described in this article to fully-control the non-desirable zinc growth patterns would require a great deal of energy supplied in the form of pumping work. Specifically, their work reports a linear flow velocity of 19.5 cm/s. As a result, the overall battery energy efficiency is likely to be very low in a practical battery, thereby limiting its application as a battery in most realistic scenarios.
Accordingly, it would be desirable to provide a means to avoid these zinc electrode related problems, such as shape change and dendritic shorting, and to provide a rechargeable nickel-zinc battery having a cycle life that is much greater than prior art nickel zinc batteries.